The present invention relates to apparatus and methods for processing substrates such as semiconductor substrates for use in IC fabrication or glass panels for use in flat panel display applications. More particularly, the present invention relates to improved techniques for controlling plasma formation in a process chamber of a plasma reactor.
The use of plasma-enhanced processes in the manufacture of semiconductor-based products (such as integrated circuits or flat panel displays) is well known. In general, plasma-enhanced processes involve processing a substrate in a process chamber of a plasma reactor. In most plasma reactors, a plasma may be ignited and sustained by supplying a gas containing appropriate etchant or deposition source gases into the process chamber and applying energy to those source gases to respectively etch or deposit a layer of material on the surface of the substrate. By way of example, capacitive plasma reactors have been widely used to process semiconductor substrates and display panels. In capacitive plasma reactors, a capacitive discharge is formed between a pair of parallel electrodes when RF power is applied to one or both of the electrodes.
Although the plasma predominantly stays in the process area between the pair of electrodes, portions of the plasma may fill the entire chamber. A plasma typically goes where it can be sustained, which is almost anywhere in the chamber. By way of example, the plasma may fill the areas outside the process region such as the bellows of the pumping arrangement. If the plasma reaches these areas, etch, deposition and/or corrosion of the areas may ensue, which may lead to particle contamination inside the process chamber, and/or which may reduce the lifetime of the chamber or chamber parts. Furthermore, a non-confined plasma may form a non uniform plasma, which may lead to variations in process performance.
Accordingly, there are continuing efforts to produce plasmas, which are confined to the process region, and thus more stable. Confined plasmas tend to ensure efficient coupling of energy to discharges, enhance plasma uniformity, and increase plasma density, all of which lead to better processing uniformity and high yields on processed substrates. There are various ways to achieve a confined plasma. One approach uses external magnetic fields to confine the plasma. Another approach uses a confinement ring to confine the plasma. The confinement ring is typically formed from an insulating material that physically blocks the plasma pumping passage, thereby confining the plasma. Both approaches have proven to be highly suitable for plasma processing, and more particularly for improving process control and ensuring repeatability. Although these approaches work well, there are continuing efforts to improve plasma confinement, and more particularly for minimizing and/or eliminating the unwanted plasma formation in the region outside of the process region of the process chamber. For example, depending on power, pressure and chemistries, a relatively strong electric field and a substantial amount of residue ionic species may be present outside the confined plasma region, which can further induce glowing discharge outside the confined process region, i.e., plasma un-confinement.